F IRE D EPARTMENT
in cooperation with
Electrical Sizing
Recommendations for Fire Pump Applications
The building code includes special requirements for generators and transfer switches supplying fire
pumps. This article captures those unique requirements and translates them into associated
equipment sizing implications.
Where a generator set supplies power to an electric fire pump there are special sizing considerations
outlined in the National Fire Protection Associations (NFPA 20) and California Electrical Code (Article
230 & 695).
The generator feed to a fire pump is typically one of two circuit arrangements. One arrangement uses
a transfer switch integral to a fire pump controller.
The second arrangement uses a listed fire pump transfer switch separate from a fire pump controller.
This sizing recommendation covers sizing the generator set for either arrangement and sizing the
transfer switch for the second arrangement, where separate from the fire pump controller.
Sizing the generator set
Background: NEC 695-7 requires that voltage dip no more than 15% of rated controller voltage at the
fire pump controller line terminals (includes cable drop) during normal starting of the fire pump motor.
This may translate to over sizing the generator set by a factor of two or three times to provide
required motor starting kVA compared to when a 30%-35% starting voltage dip is permitted.
Where the fire pump is the only significant load on the generator set, the starting kVA required will be
much greater than the required running kVA. Since there are practical limits to the alternator capacity
in a generator set, a larger genset may be required, resulting in a light load running condition for the
engine (less than the recommended minimum of 30% of rated kW). To alleviate this, consider adding
additional loads with low starting requirements, such as lighting, or the application of supplemental
load banks, especially during normal routine system testing.
All fire pump controllers, whether reduced-voltage or DOL (direct-on-line), full voltage, include an
emergency manual mechanical means to start the fire pump under full voltage should the starting
circuit or contactor coil malfunction. The exception to NEC 695-7 states that the 15% voltage dip limit
does not apply when using manual starting emergency means.
Note: It is not necessary to size the generator set for locked rotor current continuously.
Sizing the utility circuit breaker, CB1 (or fuses)
Size any over current device upstream of the fire pump controller on the utility line side to hold locked
rotor current of the fire pump motor continuously, typically a minimum of 600% of motor FLA (Full
Load Amps). Because the maximum allowable current-limiting fuse for a given size transfer switch is
higher than the maximum allowable molded case circuit breaker, using current-limiting fuses in lieu of
a circuit breaker may allow a smaller transfer switch to be used.
Sizing the feeder conductors
Sizing the feeder conductors at a minimum of 125% of the motor full load current or next higher
ampacity. Feeder conductors run from the circuit breaker at the generator (CB2) to the fire pump
controller line terminals and from the load side of CB1 to the fire pump controller line terminals.
The voltage drop requirement of NEC 695-7 also applies, so if the motor is large and the run is long,
the feeder conductors may require over sizing. The facility designer is responsible for cable drop
calculations.
Sizing the automatic transfer switch
1. Initially, size the ampere rating of the transfer switch to be equal to or next size greater than
the required feeder conductors.
2. Verify that the over current device used on the utility line side, CB1, does not exceed the
maximum allowable circuit breaker or fuse size allowed for the transfer switch. If it does,
increase the transfer switch rating to one that includes CB1 as an allowable upsteam breaker.
Sizing the generator circuit breaker, CB2
The objectives for sizing and selection of this over current device are:
A) Complying with code requirements,
B) Using a standard automatic molded case circuit breaker,
C) Selectively coordinating this breaker with locked rotor protection within the fire pump controller,
and
D) Having sufficient available fault current from the generator to clear a faulted fire pump circuit
without opening other branches of the generator supplied emergency system.
The circuit breaker should be a standard molded case circuit breaker; magnetic-only breakers and
non-auto-matic molded case switches are not recommended.
A magnetic-only (instantaneous trip) circuit breaker is not recommended. These breakers are UL
Component Recognized, but not UL Listed devices. They are only suitable for use in a UL listed
assembly, and are typically included with overloads as part of a UL listed combination motor started.
They are not UL listed for feeder conductor protection.
A non-automatic molded case switch with integral high instantaneous self-protection is not
recommended. If the fire pump circuit is faulted, the generator may have insufficient available fault
current to trip the switch. If the fire pump branch is not interrupted during a fault, an upstream device
may trip, leaving other emergency branches without power.
1. Size molded case breaker CB2 greater than 125% but less than 250% of the motor full load
current.
NFPA 20, 6-6.5, requires this breaker to pick up the instantaneous load. NEC 695-6(d)
prohibits overload protection, but requires short circuit protection. With a minimum rating of
125%, by exclusion, the breaker is not providing overload protection according to NEC 430-32.
With a maximum rating of 250%, for the breaker, by definition, qualifies as short-circuit
protection.
2. Within the range of 125% to 250%, select the smallest over current device that will allow pump
motor locked rotor current to flow longer than the 20 seconds allowed by the fire pump
controller integral protection.
This information is for reference only and is not inclusive to all of the applicable codes. For more
detailed information and requirements please refer to the following codes
•
2010 California Electrical Code
•
2007 NFPA 20 Standard for the installation of stationary pumps for fire protection
•
2010 California Fire Code
•
2010 NFPA 13 Standard for the installation of sprinkler systems
•
2010 NFPA 72 Fire Alarm Code
•
For Division of State Architect Projects refer to their website
o http://www.dgs.ca.gov/dsa/Resources/IRManual.aspx
•
For any interpretation of regulations such as IR E-3